Heat pumps work by moving heat or cold from a supply side to a demand side of the heat pump. The use and operation of heat pumps are described in U.S. Pat. Nos. 6,318,095, 5,782,101, 4,394,816, and 4,240,269 (collectively incorporated by reference herein). Heat pumps move the heat and cold by a compressor and a coil configuration using a refrigerant that is compressed and then moved through a cooling coil where air is blown through the coil to remove heat from the coil. In the wintertime, the coil emits heat from the atmosphere into the interior. In the summer, heat absorbed by the coil from the interior of the structure is exhausted through the outside unit of the heat pump. This type of configuration and its ability to change to which side the heat is exhausted makes a heat pump a popular appliance for efficient heating and cooling in any moderate climate.
The efficiency achieved by a heat pump is limited by the outside air temperature. As the outside air temperatures reach the limits of the heat pump, the amount of energy that is consumed increases and there is a point where the heat pump unit can no longer satisfy the demand. This point is near approximately a 25 degree F. outside differential, i.e., when there is more than a 25 degree F. difference between the inside and outside temperatures, the heat pump unit must operate all of the time. Outside this temperature differential in the winter, auxiliary heat is used, for example, by an electric furnace, and energy consumption is increased. In the summer time, the heat pump cannot keep up with demand and there is usually no auxiliary type of cooling for the structure. Thus, it is often not possible to only use a heat pump for heating and cooling needs beyond the moderate climatic regions. This generally limits the use of a heat pump to only the moderate range of climates in the world.
Many efforts to expand the usable range of heat pumps have resulted in innovations such as ground source heat pumps that use a much more stable heat sink, the ground, and still save energy. The expense of putting the coil under ground has been a disadvantage to the widespread use of these systems. Since the ground varies by only 20 degrees F. at the depths typically used in a ground source heat pump, savings in energy are achieved. However, the savings has not offset the cost of the system and higher maintenance costs. These systems tend to have many more problems than a typical heat pump and have only limited appeal. These systems also do not take advantage of the better temperature differences at different times of the day through other air supply sources, and thus they are not as efficient as possible.